Attempts to transfer data more efficiently than the traditional packet switching methods (e.g., X.25) have led to the development of a packet switching technique called frame relay. A frame relay network provides a number of interconnected nodes which are capable of receiving data frames from other network nodes and forwarding those data frames through to other network nodes to an ultimate destination. Nodes are interconnected by transmission paths, each of which supports one or more virtual circuits. Communication from one user to another can thus be made using the pre-defined network connections of the virtual circuits.
Frame relay networks support two types of virtual circuits: permanent virtual circuits (PVCs) and switched virtual circuits (SVCs). With a PVC service, the virtual circuit endpoints route through the frame relay network and PVC identifiers are determined when a user subscribes to the network. PVCs then are similar to leased lines in the sense that the physical connection is always present; call setup and teardown is implemented via commands issued by a network management system. SVCs have been referred to as virtual circuits "on demand". With SVCS, service requests are made on a signalling channel and the identifiers for the virtual circuit to be used during a network call are assigned at the time the call is established.
Users generally connect to a frame relay network through leased lines. The line originates at customer premises equipment (CPE) and terminates at a port on a node of the frame relay network. Although these private line connections allow data to be transferred very quickly between the CPE and the network, the lease costs can be high. For many users, the amount of data to be transferred via the frame relay network does not justify the cost of a dedicated private line. For example, remote offices that need to transmit data to a regional or national headquarters may only need to do so once per day, or even less frequently. Also, individual users may only need to access their office database occasionally and/or from remote locations (where leased lines may not be available).
On the other hand, many users transmit a significant amount of data via the frame relay network and cannot afford to lose their connection. Such users often are forced to lease multiple access lines to ensure that backup connections are available in case the primary connection to the network fails. These additional lines further increase the cost of frame relay network access for such users.